1. Field of the Invention
This invention relates to a filter circuit utilizing a surface acoustic wave filter device (hereinafter referred to as a SAW filter device) and more particularly, a filter circuit suitable for a band-pass filter connected to an amplifier for a vestigial side band system (hereinafter referred to as VSB) of a TV broadcast transmitter requiring an ideal frequency response.
2. Description of the Prior Art
A conventional band-pass filter circuit for a VSB comprises a plurality of tuning circuit elements each of which has an inductor and a capacitor. However, a great deal of time is spent in adjusting the value of inductance and capacitance of such elements so that the filter circuit elicits a predetermined frequency response. Further, the band-pass filter circuit is costly because it requires many tuning circuit elements to improve its frequency response.
In recent years, instead of the filter circuit utilizing the tuning elements, a SAW filter device is applied to the band-pass filter circuit.
A SAW filter circuit utilizing a SAW filter device presents an excellent frequency response, however the SAW filter device has a large amount of internal capacitance existing in the input electrode and output electrode. The internal capacitance is not desirable because it acts as a reactance load increasing the insertion loss. In order to minimize the insertion loss, an inductance element which cancels the capacitance of the SAW filter is connected in parallel with an output electrode of the SAW filter device. However the total frequency response of a SAW filter circuit deteriorates because of the inductance element. Thus, the ordinary SAW filter is not utilized for a VSB filter circuit.